Various operating voltages are needed in electronic systems. Usually these operating voltages are generated by so called power supplies. Generally, power supplies are divided into so called linear power supplies and so called switch-mode power supplies. Power semiconductors are used in linear power supplies in so called linear state, i.e. both the current and the voltage affect simultaneously across them. The values of the current and the voltage change linearly when moving from one operating point to another. The advantages of the linear power supplies are fast response and accurate adjustment, by which e.g. accurate and fast adjustment of the output voltage is achieved. However, as a disadvantage they have high dissipation i.e. low efficiency. The dissipation is directly proportional to the product of the voltage and the current, which simultaneously affect across a power semiconductor. In the switch-mode power supplies, the power semiconductors are driven between two extreme points, i.e. the switch is totally open or totally close, in which case the voltage and the current do not influence simultaneously across it, whereby the dissipation remains low. However, in the switch-mode power supplies it also necessary to use inductive components for coupling between different voltage levels, which causes slowness because the current of inductive components, such as an inductor, cannot change unlimitedly fast.
Usually, the operating voltages of the electronic devices are constant. In some particular applications, such as a radio transmitters having good linearity, a fixed operating voltage nevertheless causes poor efficiency. This is because power dissipation is generated in the output stage, the dissipation of which is proportional to the difference between the fixed operating voltage and the transient output voltage. In another words, the output stage generates a desired output voltage by converting the additional voltage into dissipation. This kind of arrangement creates a low total efficiency, and lately methods have been evolved to improve this. One method of this kind is so called “envelope tracking” method, in which the voltage of the power supply output stage is controlled to correspond to the respective transient output voltage, in which case the power converted into the dissipation in the output stage gets smaller and the efficiency increases, which is a commonly desired characteristic. This kind of envelope tracking power supply is usually formed by connecting in parallel a switch-mode converter, which has a function to produce needed current with a good efficiency, and linear power supply, which has a function to take care of an accurate regulation of the output voltage. Since the main energy is handled with the switch-mode converter, the efficiency of the total system is high. In practice, the limited speed of the switch-mode converter causes an upper limit for the frequency of an alternating current component of efficiently generated electricity. In other words, above of the frequency of the certain generated electricity the linear power supply has to generate the most of the electricity, in which case the total efficiency is low. For example, circuits for generating electricity including a 20 MHz alternating current component have been disclosed in literature so that the total efficiency has decreased even under 50%. Considering particular radio transmitters, such as so called OFDMA transmitters (Orthogonal Frequency Division Multiple Access), it would be preferable that by envelope tracking circuit it would be possible to generate electricity with 60 MHz alternating current component with good efficiency.
In addition, electricity of various frequencies is needed in multiple applications, such as audio applications, i.e. music reproduction and electrical amplification, as well as, for example, in producing so called gradient field of magnet resonance imaging equipment. In these applications the prior art solutions cannot either generate adequate high frequency electricity components with sufficiently high efficiency.
It can be generally stated that the known methods for generating electricity are not able to generate electricity including components of sufficiently high frequency, with sufficiently high efficiency.